Studies of mutation of radiation in bacteria are proposed. A specific mutation process will be studied preferentially: the production of class 2 de novo or converted suppressor mutations by ultraviolet radiation. These two mutation examples are thought to result from directed error by a T-C pyrimidine dimer in the transcribed strand or the non-transcribed strand of the target gene DNA, respectively. The study will be developed by investigations at three levels in the molecular physiology of the cell. The directed error hypothesis will be tested explicitly to confirm the role of a T-C dimer as the causative premutational photoproduct, in cells lacking uracil-DNA glycosylase activity and by action spectra data. The role of additional radiation-induced proteins in this mutation process will be determined with two-dimensional electrophoresis, exploiting our recent observation of crowding depression of mutagenesis so that effects on specific proteins may be related to inhibition of mutagenesis. The role of DNA replication and repair will be explored in studies of suppressor mutation at target genes carried in small plasmid replicons. The plasmid-borne target genes and their repair will be characterized by techniques using restriction endonucleases and slab gel electrophoresis. Throughout, the bacteriological consequences of mutagenesis producing revertant colonies will be refined with phage testing to determine the production of specific revertant types.